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Published on the web May 29, 2013
Preparation and Valence Tautomeric Behavior of a Cobalt-Dioxolene Complex
with a New TTF-functionalized Phenanthroline Ligand
Shinji Kanegawa, Soonchul Kang, and Osamu Sato*
Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580
(Received February 15, 2013; CL-130118; E-mail: sato@cm.kyushu-u.ac.jp)
In this study,
a new tetrathiafulvalene-functionalized
phenanthroline ligand (TTF-phen) and a mononuclear cobalt(II)
complex with a redox active 3,6-di-tert-butylsemiquinonato
(3,6-dtsq) ligand was synthesized. Magnetic measurements
and IR spectroscopy suggested that the Co complex [Co(3,6-
dtsq)2(TTF-phen)] exhibited thermally induced valence tauto-
merism in the temperature range 130-400 K. This complex is the
first example of a valence tautomeric complex with a TTF-
functionalized ligand.
The development of multifunctional materials, in which
more than two properties interact in a compound is one of the
most attractive research fields in molecular-based materials.1
Some hybrid molecular systems constructed with magnetic and
conducting molecules, have been reported to exhibit interesting
multifunctional behavior such as the coexistence of ferromag-
netism and metallic conductivity,1a and field-induced super-
conducting transitions.1b However, the control of the electrical
properties in hybrid materials by external stimuli is still
challenging. Spin-crossover (SCO) complexes are among the
most widely evaluated magnetic molecules with switching
properties,2 and some SCO-based hybrid materials with con-
ductive moieties have been reported.3 In these materials, the
coupling of a spin transition and electric conductivity is
understood to be mainly due to a chemical pressure effect in
the crystal lattice following a structural change caused by SCO.
On the other hand, valence tautomeric complexes are another
class of magnetic molecules that exhibit thermal and photo-
induced switching properties.4-7 A large number of examples of
mononuclear, dinuclear,6 and polynuclear7 cobalt complexes
that exhibit valence tautomerism have been synthesized, and
their magnetic and photomagnetic properties have been reported.
Typical mononuclear cobalt valence tautomeric complexes
exhibit electron transfer between the metal center and a
Scheme 1. Synthesis of the TTF-functionalized ligand (TTF-
phen) and its mononuclear cobalt complex [Co(3,6-dtsq)2(TTF-
phen)].
coordination moiety are connected though a conjugated ³
system, they exhibit a through-bond interaction. In fact, a
through-bond electronic interaction has been reported as an
intramolecular charge transfer that is induced by the metal ion in
acetylenic-, ethylenic-, and imine-conjugated TTF-³-pyridine-
type molecules.9,10 Although an example of SCO complexes
with TTF functional ligands has been reported by Oshio and co-
workers,11 to the best of our knowledge, there has been no report
on the observation of valence tautomerism in complexes with
³-conjugated TTF ligands. In this paper, we report the synthesis
and crystal structure of a new imine-conjugated TTF ligand and
the magnetic properties of a cobalt complex.
The synthesis of TTF-phen was performed, as shown in
Scheme 1, via an aza-Wittig reaction between formyltetrathia-
fulvalene and iminophosphorane (PPh3-phen). PPh3-phen was
prepared from 5-amino-1,10-phenanthroline and an excess
amount of dibromotriphenylphosphorane, (PPh3Br2) according
to a modified literature procedure.10a All compounds were
readily purified by precipitation or recrystallization and have
been fully characterized. The cobalt complex was first obtained
as a deep blue-black powder via the reaction of TTF-phen,
dicobalt octacarbonyl, and 3,6-di-tert-butyl-o-quinone (3,6-dtbq)
in methylcyclohexane. The powder was purified by recrystalli-
zation from AcOEt/heptane, and the cobalt complex was
obtained as fiber-like microcrystals. Unfortunately, the obtained
crystals of the complex were too small for crystal structure
analysis. The air-dried crystals were checked by thermo-
gravimetry (TG) and elemental analysis. The results of TG
(Figure S-1)12 and elemental analysis allowed us to safely
conclude the formation of the pure cobalt mononuclear complex
[Co(3,6-dtsq)2(TTF-phen)] without any solvent molecules in the
crystalline lattice. The synthetic details are presented in the
Supporting Information.12
redox-active dioxolene ligand,
a
process expressed as
[CoII-HS(dtsq)2(L)] ꢀ [CoIII-LS(dtcat)(dtsq)(L)] (L is a bidentate
N-donor ligand: 2,2¤-bipyridine, 1,10-phenanthroline, or
N,N,N¤,N¤-tetramethylethylenediamine; dtsq and dtcat are 3,5-
or 3,6-di-tert-butylsemiquinonato and di-tert-butylcatecholato,
respectively; HS: high spin, LS: low spin). Note that not only the
spin state but also the valence state of the central Co ion changes
between 2+ and 3+ in this thermal and/or photoirradiation-
induced process. Because there is such a large dynamic change
in the electronic state, we believe that valence tautomeric
complexes are a more suitable class of molecules than SCO
complexes for developing multifunctional materials. In order to
achieve interaction between the conducting ³ electrons and
paramagnetic d electrons in hybrid molecular systems, a large
number of metal complexes based on tetrathiafulvalene (TTF)
derivatives have been synthesized.8 When TTF and the
An ORTEP view of the TTF-phen ligand with atomic
numbering is shown in Figure S-2.12,13 TTF-phen is crystallized
Chem. Lett. 2013, 42, 700-702
© 2013 The Chemical Society of Japan